Although induction of contact skin sensitivity by low molecular weight 2,4-dinitrobenzenes requires the formation in vivo of 2,4-dinitrophenyl-proteins, analogous protein conjugates prepared in vitro are unable to induce this hypersensitive state. Low molecular weight 2,4-dinitrobenzenes are concentrated by isolated lymph node cells, but a representative 2,4-dinitrophenyl-protein conjugate (2,4-dinitrophenyl-bovine serum albumin) was not taken up to a detectable extent by these cells. It is inferred that there exist large quantitative differences in the extent to which dinitrophenyl-proteins are localized within cells following the administration to an intact animal of (a) those simple dinitrobenzenes which are both concentrated by lymph node cells and have the capacity to form protein conjugates in vivo, and (b) 2,4-dinitrophenyl-protein conjugates prepared in vitro. It is suggested that this difference could account for the fact that a varietyof 2,4-dinitrophenyl-proteins prepared in vitro are unable to induce contact skin sensitivity.

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